Biochemical Hydrogen Sulfide Removal Technology

Source : https://www.hydrocarbonengineering.com/special-reports/01042016/nature-at-its-finest-part-one-2915/

1. Introduction: the significance of hydrogen sulfide removal and an overview of biochemical technology

Hydrogen sulfide (h2s) is recognized as a highly noxious industrial gas that poses significant risks to human health, the environment, and infrastructure.¹ its presence in various gas streams, including natural gas, biogas, refinery gas, synthesis gas, and landfill gas, necessitates its removal before utilization.¹ even at low concentrations, H₂S can cause an irritating, rotten egg smell, and exposure limits are quickly exceeded at slightly higher concentrations, leading to severe health consequences.¹ furthermore, the combustion of gases containing H₂S results in the emission of sulfur dioxide (so2), a major contributor to acid rain and a cause of serious damage to vegetation and constructions.⁴ the corrosive properties of H₂S in moist environments can also lead to significant damage to pipelines and equipment, increasing operational costs and safety risks.³ consequently, stringent gas quality requirements and environmental regulations worldwide mandate the efficient removal of H₂S from various industrial gas streams.⁹

In response to the challenges associated with H₂S removal, a variety of technologies have been developed over the years. Among these, the biochemical process stands out as a biotechnological solution for removing H₂S from gaseous streams by absorption into a mild alkaline solution, followed by the oxidation of the absorbed sulfide to elemental sulfur by naturally occurring microorganisms.¹ invented and developed by paques in cooperation with universities, biochemical has become a go-to technology for biological H₂S removal, with applications spanning wastewater treatment, biomethane production, and the oil & gas industries.⁹ the process uniquely combines fast chemical absorption with a robust biological recovery of sulfur, offering a cost-effective and sustainable approach to gas purification.⁹ its broad applicability extends to almost any gas containing H₂S, including biogas, syngas, co2-rich off-gases, vent streams, and geothermal gas, and it can be integrated with various anaerobic wastewater treatment or digestion systems.¹ the development of biochemical through collaboration between academia and industry highlights a commitment to both scientific rigor and practical application in addressing the critical issue of H₂S pollution.

2. The biochemical mechanism of hydrogen sulfide removal in the biochemical process

The biochemical (bio)gas filtration system employs a two-stage process for the removal of hydrogen sulfide: initial chemical absorption followed by biological oxidation and sulfur recovery.⁹ in the first stage, H₂S -containing gas is brought into contact with a washing solution in an absorber column, typically using a countercurrent exchange to maximize contact efficiency.¹ this washing solution is mildly alkaline, usually containing sodium hydroxide (NaOH) or sodium carbonate (Na₂CO₃), which facilitates the dissolution and subsequent reaction of H₂S to form bisulfide ions (hs⁻).¹ the rapid chemical absorption of H₂S into the alkaline solution is a key feature, enabling the process to handle high inlet concentrations effectively.

The sulfide-rich washing water is then transferred to a bioreactor, where the second stage of the process occurs.¹ in this bioreactor, naturally occurring sulfur-oxidizing bacteria (sob) biologically oxidize the dissolved sulfide (hs⁻/s²⁻) to elemental sulfur particles (s⁰).¹ this biological oxidation is typically an aerobic process, requiring the introduction of air or oxygen into the bioreactor.¹ a notable advantage of the biochemical system is that air or oxygen is not directly added to the biogas stream, thus maintaining its calorific value and quality.⁹ the biological conversion of sulfide to elemental sulfur simultaneously regenerates the alkaline washing solution, allowing it to be continuously recycled back to the absorber column, which significantly reduces operational costs associated with chemical consumption.⁷

The final step in the biochemical process involves the separation of the elemental sulfur produced in the bioreactor.¹ this is typically achieved using a solids separation step, such as gravity settling in a settler or mechanical separation using a decanter or centrifuge.¹ the separated elemental sulfur is of high quality and can be reused as an organic fertilizer, further enhancing the sustainability of the process.¹ the integration of chemical absorption with biological oxidation and sulfur recovery in a closed-loop system underscores the efficiency and environmental benefits of the biochemical technology.

3. Key microorganisms and their metabolic pathways in biochemical

The biochemical process relies on the activity of naturally occurring sulfur-oxidizing bacteria (sob) to achieve the conversion of dissolved sulfide to elemental sulfur.¹ in the shell-paques/biochemical variant, a mixed culture of sob is employed.³ key genera involved in the biochemical process include thiobacillus and thioalkalivibrio.² specifically, neutrophilic thiobacillus species such as thiobacillus denitrificans and thiobacillus thioparus have been identified as active in removing H₂S under weakly alkaline conditions.³ furthermore, thioalkalivibrio species are particularly relevant due to their ability to thrive under haloalkaline conditions, which are characteristic of the biochemical process.¹ the selection and adaptation of these naturally occurring and robust bacteria are crucial for the stable and efficient operation of the technology.

The metabolic pathway for H₂S oxidation in these bacteria typically involves the aerobic oxidation of sulfide (hs⁻) to elemental sulfur (s⁰) under oxygen-limited or microaerophilic conditions.³ oxygen serves as the primary electron acceptor in this process.¹ while the specific enzymatic reactions are complex and might vary slightly between different species, the overall pathway results in the production of elemental sulfur and hydroxide ions (oh⁻).² this generation of hydroxide ions is significant as it contributes to the regeneration of the alkaline washing solution in the absorber. However, under certain conditions, such as sulfide-limited or oxygen-rich environments, the bacteria can further oxidize sulfide to sulfate (so₄²⁻).¹ additionally, intermediate sulfur compounds like thiosulfate (s₂o₃²⁻) and polysulfides (sₓ²⁻) can be formed during the oxidation process.³ understanding the specific metabolic capabilities of the microorganisms involved and the conditions that influence their activity is essential for optimizing the biochemical process to maximize elemental sulfur production and minimize the formation of undesirable byproducts.

4. Chemical reactions and intermediate compounds formed during biochemical treatment

The biochemical removal of hydrogen sulfide using the biochemical® technology involves a sequence of chemical and biological reactions. In the absorber column, the primary reactions are the absorption of h2s gas into the alkaline washing solution. This process begins with the physical dissolution of h2s into the aqueous phase, followed by chemical reactions with hydroxide and carbonate ions.² the key reactions include:

●       Dissolution of hydrogen sulfide: H₂S (g) ⇌ H₂S (aq)

●       Reaction with hydroxide: H₂S (aq) + OH⁻(aq) ⇌ HS⁻(aq) + H₂O(l) ²

●       Reaction with carbonate: H₂S (aq) + co₃²⁻(aq) ⇌ HS⁻(aq) + HCO₃⁻(aq) ²

The use of an alkaline solution, typically maintained by the addition of NaOH or Na₂CO₃, is crucial for driving these absorption reactions to the right, effectively capturing H₂S from the gas stream.¹ in cases where the gas stream contains carbon dioxide (co2), additional reactions with hydroxide ions can occur:

●       Absorption of carbon dioxide: co₂(g) + OH⁻(aq) ⇌ HCO₃⁻(aq) ²

●       Formation of carbonate: HCO₃⁻(aq) + OH⁻(aq) ⇌ CO₃²⁻(aq) + H₂O(l) ²

These reactions contribute to the buffering capacity of the solution, helping to maintain the desired pH range for optimal H₂S absorption.

In the bioreactor, the absorbed bisulfide ions (HS⁻) are biologically oxidized by sulfur-oxidizing bacteria. The main biochemical reaction is the oxidation of bisulfide to elemental sulfur, represented as:

●       2 HS⁻(aq) + O₂(aq) → 2 S⁰(s) + 2 OH⁻(aq) ³

This reaction highlights the crucial role of oxygen, supplied through aeration, and the production of elemental sulfur as a solid precipitate. The simultaneous generation of hydroxide ions regenerates the alkalinity of the washing solution, which is then recycled to the absorber. Besides the main reaction, several side reactions can occur in the bioreactor:

●       Complete oxidation to sulfate: HS⁻(aq) + 2 O₂(aq) → SO₄²⁻(aq) + H⁺(aq) ³

●       Chemical oxidation to thiosulfate: HS⁻(aq) + O₂(aq) → ½ S₂O₃²⁻(aq) + ½ H₂O(l) ³

●       Formation of polysulfides: HS⁻(aq) + (x-1)S⁰(s) ⇌ Sₓ²⁻(aq) + H⁺(aq) ³

●       Oxidation of polysulfides: Sₓ²⁻(aq) + ½ O₂(aq) → S₂O₃²⁻(aq) + (x-2)S⁰(S) ³

The key intermediate compounds formed during the biochemical® process include bisulfide ions (hs⁻), which are the primary form of dissolved sulfide in the alkaline solution, and polysulfide ions (sₓ²⁻), which can form through the reaction of bisulfide with elemental sulfur.³ while not always detailed, sulfite (so₃²⁻) might also be an intermediate in the pathway to sulfate formation. The final desired product is elemental sulfur, which is recovered as a solid.¹ undesirable byproducts such as sulfate (so₄²⁻) and thiosulfate (s₂o₃²⁻) can also be formed, impacting the overall efficiency and requiring potential management of the liquid effluent.¹

5. Impact of operational parameters on biochemical process efficiency

Several operational parameters significantly influence the efficiency of the biochemical process. Maintaining the appropriate pH is critical for both the chemical absorption of H₂S and the biological activity of the sulfur-oxidizing bacteria. The typical pH range for the biochemical system is between 8 and 9, or specifically 8.2 to 9.¹ this mildly alkaline environment ensures efficient conversion of H₂S gas to bisulfide ions in the scrubber.¹ the presence of a carbonate/bicarbonate buffer system helps to stabilize the pH within this optimal range, even with variations in the incoming gas composition.⁷

Temperature also plays a role in the biochemical process. Generally, the system operates at ambient temperature, which is an energy-efficient aspect.¹ however, as a biological process, the metabolic activity of the bacteria is temperature-dependent.¹⁶ while specific optimal temperature ranges are not consistently provided across all sources, some applications might utilize heat exchangers to maintain a suitable temperature, such as around 35°c-37°C, to ensure optimal bacterial performance.¹⁶

Nutrient availability is another crucial factor for the health and activity of the sulfur-oxidizing bacteria in the bioreactor.²  Technology Supplier typically supplies proprietary nutrients that are continuously fed into the bioreactor to sustain the microbial population.⁷ the specific composition of these nutrients is usually kept confidential but is essential for maintaining high H₂S removal efficiency.

Other operational parameters that affect the process include alkalinity and conductivity. Maintaining alkalinity in the range of 700-800 meq/l and conductivity between 40 and 55 ms/cm has been associated with stable operations and H₂S removal efficiencies exceeding 95%.¹ the supply of oxygen to the bioreactor must be carefully controlled. Insufficient oxygen can limit the oxidation of sulfide to elemental sulfur, while excessive oxygen might promote the undesirable formation of sulfate.¹ the hydraulic retention time (HRT) in the bioreactor influences the extent of sulfide oxidation, with longer HRT’s potentially leading to more complete conversion. Finally, while the biochemical system is known for its ability to handle fluctuations in gas flow rate and H₂S inlet concentration ⁹, extreme variations can still impact the overall efficiency. Careful monitoring and control of these interconnected parameters are vital for ensuring the stable and efficient performance of the biochemical technology.

6. Advantages and disadvantages of biochemical technology compared to other H₂S removal methods

The biochemical technology offers several compelling advantages over traditional hydrogen sulfide removal methods. One of the most significant benefits is its high H₂S removal efficiency, consistently achieving treated gas concentrations below 100 ppm, even with inlet concentrations ranging from 1,000 to 200,000 ppm.⁹ removal rates often exceed 99%.¹ the process is also highly cost-effective due to the biological regeneration of the alkaline washing solution, which minimizes the consumption of expensive chemicals; only modest amounts of caustic and nutrients are required.¹ unlike some other technologies, biochemical operates without the need for expensive catalysts or high temperatures and pressures, further reducing operational costs.¹

From an environmental perspective, biochemical is a sustainable technology. It produces elemental sulfur as a byproduct, which can be reused as a high-quality organic fertilizer, minimizing waste.¹ the process generates minimal hazardous byproducts.⁵ biochemical has a proven track record with over 30 years of operational experience and hundreds of installations worldwide, demonstrating high reliability and uptime, often exceeding 98% or 99%.² the system is also flexible, capable of handling large fluctuations in gas flow rates and h2s inlet concentrations.² a key advantage for biogas applications is that biochemical® does not introduce air or oxygen directly into the biogas stream, thus maintaining its high calorific value, which is crucial for use in gas engines or for upgrading to biomethane.⁶ operationally, the process is relatively simple with minimal supervision and control requirements compared to more complex chemical methods.¹ the biologically produced sulfur is hydrophilic, which reduces the risk of equipment fouling and plugging, a common issue in some other liquid-based processes.² biochemical boasts broad applicability across various gas streams and industries ¹, and it enhances safety by operating at ambient temperature and pressure in the regeneration and sulfur recovery sections, with essentially no free H₂S present downstream of the scrubber inlet.²

Despite its numerous advantages, biochemical also has some potential drawbacks. As a biological system, it can be sensitive to significant fluctuations in operational parameters such as pH, temperature, and nutrient supply, requiring careful monitoring and control to maintain optimal performance.¹ the start-up time for a biological system might be longer compared to purely chemical methods, as the microbial community needs time to establish and reach optimal activity.¹ while generally reliable, there have been reports of potential issues like sulfur clogging or foam formation in some applications, although these can often be mitigated with proper design and operational adjustments.¹ compared to purely chemical methods, biochemical might have limitations in rapidly responding to sudden and extreme changes in H₂S loading, which could be a concern in processes with highly variable gas compositions.²⁹ while operating costs are low, the initial capital investment for a biochemical system might be higher than some simpler technologies, particularly for very small-scale applications.³⁰ finally, although the process is highly selective for elemental sulfur production, the formation of byproducts like sulfate or thiosulfate is possible and might require management of the effluent stream.¹

When compared to other H₂S removal technologies, biochemical offers distinct advantages. Compared to the traditional amine/claus process, biochemical boasts a simpler design with fewer control requirements and potentially lower costs for sulfur loadings up to a certain threshold.¹ against liquid redox processes like lo-cat and sulferox, biochemical eliminates the need for expensive and potentially hazardous chemicals, leading to reduced operating costs and enhanced safety.² in comparison to caustic scrubbing, biochemical's biological regeneration of the caustic solution results in significantly lower caustic consumption and associated costs.⁶ while iron sponge or other adsorbent media might have lower upfront costs for small applications, they often require periodic replacement and disposal, potentially increasing long-term operating costs and waste generation.²³ scavengers, although having low capital costs, are consumed in the process and can produce hazardous byproducts, leading to higher operating costs and disposal challenges.³⁰

Table 1: comparison of H₂S removal technologies

7. Case studies and real-world applications of biochemical technology

The biochemical technology has been successfully implemented in numerous real-world applications across various industries. In wastewater treatment, one of the earliest full-scale applications was at industriewater eerbeek in the netherlands, where it has been treating biogas produced from paper mill effluent since 1997.¹⁵ long-term operational data from this site demonstrate the robustness of the technology, maintaining high uptime and effectively handling fluctuations in both biogas flow and h2s concentration.¹⁵ in the united states, a 40 mgd water pollution control facility utilized a biochemical® system to treat biogas generated from anaerobic digesters processing industrial wastewater, achieving an impressive average H₂S removal efficiency of 99.8%.⁷ the technology is also employed for the treatment of landfill gas, as seen at ecopark de wierde in the netherlands, where a biochemical® scrubber significantly reduced H₂S levels in the extracted gas, making it suitable for use in gas engines.¹⁵ additionally, biochemical® has been applied to treat biogas generated from the anaerobic digestion of biosolids.¹⁵ these case studies highlight the versatility and reliability of the technology in managing H₂S in the biogas produced during wastewater treatment processes.

In the realm of biogas purification for energy production, biochemical plays a critical role in enabling the utilization of biogas as a renewable energy source. It is widely used to purify biogas for use in combined heat and power (CHP) units, ensuring that the gas meets the required quality standards to prevent corrosion and damage to the engines.¹⁵ furthermore, with the increasing focus on sustainable transportation fuels, biochemical is instrumental in the upgrading of biogas to biomethane, which can be used as vehicle fuel or injected into the natural gas grid.⁹ the technology's ability to achieve deep H₂S removal ensures that the upgraded biomethane meets the stringent gas quality specifications for these applications.⁹

The oil and gas industry has also adopted the biochemical technology, particularly the shell-paques process, for the desulfurization of various gas streams, including natural gas, synthesis gas, and refinery gas.¹ this variant of the technology is capable of handling high-pressure applications, up to 75 barg (approximately 1300 psi).² a notable case study involves an independent oil and gas operator in the us who utilized biochemical to treat sour casinghead gas with a very high H₂S concentration of 40,000 ppm, successfully reducing it to below 4 ppm in the treated gas and enabling the recovery of high-quality LPG.¹⁴ The biochemical process has also been applied for sulfur recovery from spent sulfuric acid streams in the oil and gas sector.²

Beyond these primary sectors, the biochemical process has found applications in other industries as well. It has been commercialized for sulfur removal in the pulp and paper, chemical, and mining industries.² there is also potential for its use in purifying geothermal gas, which often contains significant amounts of H₂S.⁹ the diverse range of case studies and applications underscores the adaptability and effectiveness of the biochemical technology in addressing H₂S removal challenges across various industrial contexts.

 

8. Cost-effectiveness and scalability of biochemical technology for industrial applications

The biochemical technology is recognized for its cost-effectiveness in hydrogen sulfide removal, particularly for industrial applications. The primary driver of its economic advantage is the biological regeneration of the alkaline scrubbing solution in the bioreactor.⁵ this in-situ regeneration significantly reduces the need for fresh caustic, leading to substantial savings in chemical consumption compared to conventional caustic scrubbers or liquid redox processes.⁶ in some cases, caustic consumption in the biochemical process can be as low as 5-10% of that of a traditional caustic scrubber.¹⁸ furthermore, the process operates at ambient temperature and pressure for the regeneration and sulfur recovery sections, eliminating the energy costs associated with heating and pressurization required by some other technologies like the amine/claus process.² the production of elemental sulfur as a valuable byproduct can also contribute to the overall cost-effectiveness, as it can be sold or used as an organic fertilizer.¹

The scalability of the biochemical technology for industrial applications is also a significant benefit. The process has been implemented across a wide range of gas flow rates and sulfur loads. Biogas flows treated by biochemical systems vary from 50 nm3/hour to more than 50,000 nm3/hour, and daily sulfur loads range from 10 kg/day to 50,000 kg/day.⁹ standardized system designs enable short project lead times, while experienced engineering teams can tailor units to meet specific needs.⁹ the shell-paques variant extends the scalability to high-pressure applications and higher sulfur recovery rates, up to 150 tons per day.² the technology has been successfully deployed in facilities with varying biogas production capacities, from smaller wastewater treatment plants to large industrial sites and oil & gas processing facilities.² the modular design and the availability of standardized systems contribute to the ease of scaling the biochemical process to match the requirements of different industrial applications. The continuous innovation and the extensive operational experience gained from hundreds of installations worldwide further support the reliability and scalability of the technology for diverse H₂S removal needs.²

9. Recent advancements or modifications in the biochemical technology for improved performance or wider applicability

The biochemical technology has undergone continuous development and optimization since its inception, leading to several advancements and modifications aimed at improving its performance and widening its applicability. For over 30 years, biological gas desulfurization under halo-alkaline conditions, as employed by biochemical process, has been a subject of study and refinement.²⁰ one recent advancement involves the insertion of a novel sulfidic reactor into the conventional process setup.²⁰ this modification promotes the removal of the smallest individual sulfur particles and encourages the production of larger sulfur agglomerates, which can improve process operation, sulfur separation, and sulfur recovery.²⁰ experimental and modeling results have demonstrated the potential of this addition to enhance the efficiency of the biochemical process.²⁰

Another area of development focuses on expanding the applicability of the technology to treat various gas streams under different conditions. The shell-paques process represents a significant modification that allows for the effective removal of H₂S from high-pressure gas streams in the oil and gas industry, demonstrating the adaptability of the core technology to more demanding environments.¹ research has also explored the potential of biochemical technology for treating synthesis gases, although this application might still be under further development.¹⁵ continuous innovation by paques aims to provide tailored gas treatment solutions that enable customers to achieve very low hydrogen sulfide content in their biogas at low operational costs, facilitating its use in local gas-fired microgrids or upgrading to biomethane.¹¹ these ongoing efforts to optimize the process and adapt it to new challenges and applications underscore the dynamic nature of the biochemical technology.

10. Conclusions

The biochemical hydrogen sulfide removal technology based on biochemical represents a robust, cost-effective, and environmentally sustainable solution for a wide range of industrial applications. By combining efficient chemical absorption with the regenerative power of sulfur-oxidizing bacteria, biochemical achieves high h2s removal efficiencies while minimizing chemical consumption and producing a valuable elemental sulfur byproduct. Its broad applicability across wastewater treatment, biogas purification, and the oil & gas industry, coupled with its proven reliability and ability to handle fluctuating gas flows and H₂S concentrations, positions biochemical as a leading technology in the field of gas desulfurization. Continuous advancements and modifications, such as the integration of novel reactor designs, further enhance its performance and broaden its applicability. While careful monitoring of operational parameters is necessary to maintain optimal efficiency, the advantages offered by biochemical, including lower operating costs, high uptime, and environmental benefits, make it an attractive choice for industries seeking sustainable and effective H₂S removal solutions.

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